1. Field of the Invention
The present invention relates to a differential limiting control apparatus for a four wheel drive vehicle. More specifically, the invention relates to a technology capable of controlling differential limiting force of a center the differential according to road and vehicle running conditions.
2. Description of Prior Arts
In recent years, many vehicles including four wheel drive vehicles are equipped with an anti-lock brake control system (hereinafter, referred to as ABS) to prevent sustained wheel-locking by controlling braking force on braking.
Generally, the ABS is constituted so as to control braking force by detecting wheel slippage on braking. In applying this system to four wheel drive vehicles, sophisticated control techniques are needed due to the relationship of driving force distributed between front and rear wheels.
For example, Unexamined Japanese Patent Application No. Toku-Kai-Shou 62-43355 discloses a technique in which the differential limiting of a center differential is released when depressing a brake pedal to operate the ABS.
According to the aforesaid technique, however, since the engagement of driving force between front and rear wheels is released even in case of depressing the brake pedal lightly, especially when the brake pedal is depressed repeatedly, frequent engagement and disengagement between front and rear wheels are performed and as a result a driver feels awkwardness due to the frequent changes of driving performance of the vehicle.
That is to say, the braking force applied equally to four wheels is suddenly applied more to front or rear wheels and as a result the under-steer characteristic of the vehicle becomes too strong or inversely it becomes too weak.
To solve such a problem, Examined Japanese Patent Application No. Toku-Kou-Hei 6-88504 proposes a technique wherein the differential limiting is released when a rotational deceleration of wheels exceeds a specified value on braking and after that the ABS is operated if the ABS operational condition is satisfied, so as to start the ABS control only when the wheels come closer to a locking state.
This technique still has a problem that when the rotational deceleration of wheels becomes large, the abrupt release of the differential limiting causes a sudden slip on the front or rear wheels and as a result the driver may feel a large change in the under-steer characteristic of the vehicle for an instant before the ABS control starts.
Further, for example, when an emergency braking is applied to the vehicle on a pavement having a high road friction coefficient, the wheel rotation is largely decelerated without causing slip. In this moment, since the engagement between front and rear wheels is abruptly released while a strong braking force is retained, a shock or an impact noise may be generated from the drive train of the vehicle in the moment the engagement is released.